Air traffic control officers at Ho Chi Minh City’s Tan Son Nhat Airport lost contact with aircraft for 18 minutes on Tuesday morning, forcing incoming pilots to circle or land at nearby airports, the aviation authority said.

The incident occurred at 7:47 a.m., the Civil Aviation Administration of Vietnam said on Tuesday afternoon.

Six aircraft had to fly around and wait for landing permissions while another had to land at another airport. Flights departing from the airport were delayed, the agency said.

It blamed the incident on interference to radio frequencies, adding that an investigation into the case is ongoing.

An official from the agency, who wished to go unnamed, said this is the first time Tan Son Nhat has had such an incident. He said that it was “less serious than the blackout last November.”